1. Field of the Invention
The present invention generally relates to testing electronic circuits and, more particularly, to a contactless method and apparatus for testing for opens and shorts in thin film networks connected to a bottom surface metallurgy (BSM) input/output (I/O) pad on a multi-chip module (MCM) containing electrical thin film wiring nets or laser vias under test.
2. Background Description
During the manufacture of the thin film layers of MCM substrates, it may not be possible to utilize contact test methods until all top surface metallurgy (TSM) thin film layers of a substrate are completed. For yield reasons, it may be desirable to locate shorts and opens on nets as each thin film layer is made and repair them where possible.
General shorts and opens may be detected by optical inspection techniques with some success and some amount of escapes. MCM substrates may tolerate some level of shorts and opens as many substrates are made with the possibility of adding or deleting wiring after the part is completed or almost completed. Automatic optical inspection (AOI) may be costly or may not always be effective and sometimes test methods are more reliable.
Not all shorts or opens are equally serious due to many MCM substrates being designed for repairability. In general, if a short or open occurs on a net connected to a BSM I/O pad, it would be more serious and likely a fatal defect as it would not be possible to wire around or delete it. It is therefore desirable for yield reasons to find and locate shorts and opens on thin film nets connected to BSM I/O pads with special care and as effectively as possible.
Capacitive methods have had some success for some products where a capacitive plate is placed over the part and the capacitance at each BSM I/O pad is measured and compared to a learned value. A higher or lower capacitance reading may indicate a short or open respectively using such a test. One drawback to such a test is that the capacitance will be influenced by the brick line together with the thin film pattern under test. If the brick line is very long, the percentage of the capacitance due to the thin film portion of the net may be small and it may not be practical or possible to efficiently find opens in the thin film net under test, so the capacitance approach may be effective for shorts but for very complex products may not always be as effective for opens.
Another class of fatal opens would be those which occur in the laser vias between metalization layers and, as before, if a laser via on a BSM I/O net was too small or clogged, it may be a fatal defect. A capacitive approach would be no help in testing laser vias on BSM I/O nets. Optical approaches to laser via inspection may be problematic due to the exceedingly small size of laser vias, the variability of the appearance of the surface of the metal at the bottom of the via and the need to catch small amounts of residual polyimide at the bottom of the via. In the case of laser via inspection, it may be less possible to repair a bad laser via, but it would be valuable for process diagnostics and yield learning to locate defective laser vias.
In some cases, it may not be possible to directly contact the BSM pads of an MCM substrate since for some products, the BSM is sealed in polyimide to prevent or minimize a variety of manufacturing problems and capacitive tests which require direct contact with a BSM pad may not be possible.
Some have applied plasma techniques to inspection which also offer a noncontact test to the most general classes of TSM shorts and opens. These approaches tend to drive mechanical, optical complexity and may require image processing.